WASHINGTON (AFP) - The United States believes it has killed about 6,000 militants in its air strikes against the Islamic State in Iraq and Syria (ISIS) group, defence officials said on Thursday.

Pentagon chief Chuck Hagel, however, warned against resorting to "body counts" to measure progress in the war.

The estimate of how many ISIS fighters had been taken out by US-led air raids was first cited by Washington's ambassador to Iraq, Stuart Jones, in an interview aired on Thursday by Al-Arabia television.

US defence officials reluctantly confirmed the figure but insisted the military was not placing a priority on the measurement, which carried uncomfortable echoes from the Vietnam conflict - when American commanders cited daily "body counts" to convey progress in the war.

There was no independent confirmation of the casualty estimate and it remains unclear how many civilians might have been inadvertently killed in the strikes.

But if accurate, it would suggest the coalition has inflicted astounding damage on the ISIS group since the air strikes began in Iraq on August 8 and in Syria on September 23.

The casualty toll would mean bombing raids wiped out roughly 20-30 per cent of the ISIS group's fighting force, which is estimated at between 20,000 to 31,500 fighters, according to the Central Intelligence Agency's estimates released last year.

Hagel told a news conference he could not confirm the 6,000 death toll but said "thousands" of ISIS fighters had been killed in US-led bombing raids.

"We do know that thousands of ISIL fighters have been killed, and we do know that some of ISIL's leadership have been killed," he said, referring to another acronym of the militant group.

However, Hagel, a Vietnam veteran who still carries shrapnel fragments in his chest from his war wounds, said the number of ISIS militants killed was not a useful gauge for assessing the effect of the air campaign.

"I was in a war where there was a lot of body counts every day. And we lost that war," he said.

Other indicators showed that the ISIS was on the defensive and under pressure, Hagel said, including signs that the militants were having trouble recruiting, keeping supply routes open and maintaining communications on the battlefield.

"These are the metrics you look at as to how much progress you're making in a war," he added.